Field of the Invention
The present invention relates to an aqueous ink, an ink cartridge and an ink jet recording method.
Description of the Related Art
An ink jet recording method can conduct recording on various recording media. Various kinds of inks such as, for example, an ink suitable for recording an image having photograph-like quality on glossy paper and an ink suitable for recording a document on plain paper for purpose of recording a better image have been proposed according to its purpose. In recent years, the ink jet recording method has also been utilized for printing a business document including characters and diagrams using a recording medium such as plain paper, and utilization frequency to such use has remarkably increased. In such use, it is required to more improve an optical density of an image recorded and reliability of an ink used.
It is advantageous from the viewpoint of improving the optical density of the image to use a self-dispersible pigment as a coloring material of the ink. When the self-dispersible pigment is used as the coloring material of the ink, it is favorable that the dispersion stability of the pigment in the ink is secured, and moreover the pigment is aggregated immediately after the ink is applied to a recording medium. In addition, after the ink has been applied to the recording medium, the pigment is favorably caused to exist on the surface of the recording medium as much as possible by permeating a liquid component such as water into the recording medium. Therefore, a self-dispersible pigment with an anionic group introduced in plenty into a particle surface of the pigment is favorably used. When such a self-dispersible pigment is used, the pigment can be stably dispersed in the ink by repulsive force between anionic groups. In addition, after the ink has been applied to the recording medium, the dispersed state can be unstabilized by vanishment of the repulsive force between anionic groups which is caused by evaporation of water, thereby immediately aggregate the pigment. From the viewpoint of achieving both good dispersion stability in the ink and improvement of an optical density of an image by acceleration of pigment aggregation in the recording medium, a carboxylic acid group is favorably selected as the anionic group introduced to the particle surface of the pigment.
Various type pigments are known as the self-dispersible pigment to which the carboxylic acid group has been introduced and roughly divided into an oxidation type and a surface modification type according to the production processes thereof. The oxidation type self-dispersible pigment is obtained by subjecting a pigment to an oxidation treatment with ozone gas or hypochlorous acid (see Japanese Patent Application Laid-Open No. 2003-535949). The surface modification type is obtained by chemically treating a pigment with a compound having an anionic group, such as a diazonium salt or a hydrazine compound (see Japanese Patent Application Laid-Open No. H10-510862 and Japanese Patent Application Laid-Open No. 2012-528917). There is further a self-dispersible pigment obtained by bonding a carboxylic acid group to a pigment by an oxidation treatment and then further bonding a functional group having a lactone group at a terminal thereof (see Japanese Patent No. 5085868).
When an image where a region recorded by an ink containing a self-dispersible pigment and a region recorded by another ink adjoin each other is recorded, bleeding is generally eased by the following recording control. That is, after repulsive force between anionic groups of the self-dispersible pigment has vanished, and the self-dispersible pigment has start to aggregate in a recording medium, another ink is applied, thereby preventing the self-dispersible pigment from flowing into the region recorded by another ink. However, when a region where an application amount of an ink is large, such as a multi-order color image, is recorded adjoiningly to the region recorded by the ink containing the self-dispersible pigment, a phenomenon (white blur phenomenon) in which an image looks whitish occurs different from the bleeding. This “white blur phenomenon” is a phenomenon caused by allowing the self-dispersible pigment present in the vicinity of a boundary portion between the above two regions to sink in the interior of the recording medium.
The oxidation type self-dispersible pigment described in Japanese Patent Application Laid-Open No. 2003-535949 is hard to control a chemical species introduced to the particle surface of the pigment. Therefore, a nonionic hydrophilic group such as a lactone group comes to be bonded in plenty to the particle surface of the pigment, not just the carboxylic acid group. When the lactone group is bonded in plenty to the particle surface of the pigment, the aggregation of the pigment becomes gentle due to the hydrophilicity of the lactone group even after the repulsive force between anionic groups has vanished. In addition, the self-dispersible pigment described in Japanese Patent No. 5085868 has not only a lactone group formed on the particle surface of the pigment by the oxidation treatment, but also the functional group having the lactone group at the terminal thereof, so that the aggregation of the pigment becomes gentle.
On the other hand, the surface modification type self-dispersible pigments described in Japanese Patent Application Laid-Open No. H10-510862 and Japanese Patent Application Laid-Open No. 2012-528917 are easy to control a chemical species introduced to the particle surface of the pigment compared with the oxidation type self-dispersible pigment. However, even in the self-dispersible pigments of the above-described types, a certain amount of a lactone group is bonded to the particle surface of the pigment before the treatment irrespective of the production processes of the self-dispersible pigments. Therefore, the aggregation of the pigment becomes gentle in the recording medium due to the action of the lactone group, and so a problem that bleeding or white blur is easy to occur has been caused.